Wavelength division multiplexed (WDM) optical communication systems (referred to as “WDM systems”) are systems in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber using an optical multiplexer circuit (referred to as a “multiplexer”). Such systems may include a transmitter circuit, such as a transmitter (Tx) photonic integrate circuit (PIC) having a transmitter component to provide a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and multiplexer to combine each of the modulated outputs (e.g., to form a combined output).
A PIC is a device that integrates multiple photonic functions on a single integrated device. PICs may be fabricated in a manner similar to electronic integrated circuits but, depending on the type of PIC, may be fabricated using one or more of a variety of types of materials, including silica on silicon, silicon on insulator, and various polymers and semiconductor materials which are used to make semiconductor lasers, such as GaAs, InP and their alloys.
A WDM system may also include a receiver circuit having a receiver (Rx) PIC and an optical demultiplexer circuit (referred to as a “demultiplexer”) configured to receive the combined output and demultiplex the combined output into individual optical signals. Additionally, the receiver circuit may include receiver components to convert the optical signals into electrical signals, and output the data carried by those electrical signals.
The transmitter (Tx) and receiver (Rx) PICs, in an optical communication system, may support communications over a number of wavelength channels. For example, a pair of Tx/Rx PICs may support ten channels, each spaced by, for example, 200 GHz. The set of channels supported by the Tx and Rx PICs can be referred to as the channel “grid” for the PICs. Channel grids for Tx/Rx PICs may be aligned to standardized frequencies, such as those published by the Telecommunication Standardization Sector (ITU-T). The set of channels supported by the Tx and Rx PICs may be referred to as the ITU frequency grid for the Tx/Rx PICs.
In WDM systems, the demultiplexer may be capable of receiving first and second optical signals associated with the combined output in order to increase data rates associated with the WDM system. In order to further increase the data rates associated with a WDM system, additional WDM components are sometimes incorporated. For example, some WDM systems may include a polarization beam splitter (PBS) to receive the combined output and output first and second optical signals, to increase the data rates associated with the WDM system. The first optical signal may include components having a first polarization and the second optical signal may include components having a second polarization. Some WDM systems may further include a rotator to rotate the polarization of the components associated with the first optical signal such that the components have the second polarization, thereby allowing the demultiplexer to process optical signals associated with one polarization (e.g., the second polarization).
WDM systems are sometimes constructed from discrete components (e.g., a transmitter component, a multiplexer, a demultiplexer, a PBS, a rotator, and/or a receiver component). For example, demultiplexers and receiver components may be packaged separately and provided on a printed circuit board. Alternatively, WDM components are sometimes integrated onto a single chip, also referred to as a photonic integrated circuit (PIC). For example, a PBS and a rotator are provided on the same PIC as a demultiplexer.